Types Of Lewis Acid Sites Research Articles

Investigation of the acidic nature of MCM-68 zeolite based on the adsorption of CO and bulky probe molecules

The aluminosilicate MCM-68 has an MSE topology, a unique pore system, and solid acid characteristics. For these reasons, it has potential applications as a highly efficient catalyst for paraffin cracking, methanol-to-olefin/dimethyl ether-to-olefin reactions, and the alkylation of aromatic compounds. In this work, the acidic properties of both the parent MCM-68 and dealuminated versions were evaluated using infrared spectroscopy in conjunction with the adsorption of CO and/or 2,4,6-trimethylpyridine (collidine). Notably, the relatively bulky collidine molecules could not enter the 12-ring pore-mouths of the MCM-68 at room temperature but were able to do so at 403 K, due to the excitation of pore-mouth breathing vibrations that allowed increased diffusion at higher temperatures. During CO adsorption study, two types of Lewis acid sites and one type of Brønsted acid sites were identified on the parent MCM-68. After dealumination by acid treatment, the Brønsted and stronger Lewis acid sites, especially those on the external surfaces of the MCM-68 particles, were preferentially removed.

A robust viologen and Mn-based porous coordination polymer with two types of Lewis acid sites providing high affinity for H2O, CO2 and NH3.

A novel porous coordination polymer [Mn(pc3)(H2O)2]·xH2O (3 < x < 4) is synthesized in water at pH = 7 using the anionic viologen-carboxylate ligand 4,4'-bipyridinium,1,1'-bis-(2,4-dicarboxyphenyl) (pc32-). Dehydration of the material results in the formation of open pores containing two types of accessible Lewis acid sites: exposed Mn2+ cations and N+ atoms of viologen units. Due to this property the PCP shows high affinity and capacity in the adsorption of H2O, CO2 and NH3. Despite the presence of strong adsorption sites this material is stable in liquid water and in gaseous NH3.

Ethanol Conversion to 1,3-Butadiene on ZnO/MgO–SiO2 Catalysts: Effect of ZnO Content and MgO:SiO2 Ratio

Effect of ZnO on ethanol conversion to 1,3-butadiene (BD) on ZnO/MgO–SiO2 catalysts consists in the enhancement of selectivity and yield of BD due to higher rate of ethanol dehydrogenation to acetaldehyde. It has been shown that two types of Lewis acid sites formed in the contact zones of MgO–SiO2 and ZnO–SiO2 in the catalysts play an important role in BD synthesis from ethanol. The highest BD yield is achieved at ratio of MgO:SiO2 = 1:1.

DRIFTS Study of Surface of γ-Alumina and Its Dehydroxylation

γ-Alumina surfaces pretreated at different temperatures (25 to 450 °C) and their rehydration after pyridine adsorption were studied using the conventional pyridine adsorption/DRIFTS technique. The results show that hydration at ambient temperature cannot convert all of the surface-coordinatived unsaturated Al sites to six-coordination, and weak and medium strong Lewis acid sites can still be observed for the fully hydrated γ-alumina. Dehydration at above 200 °C starts to generate strong Lewis acid sites. The number of weak, medium strong, and strong Lewis acid sites changes with dehydration temperature and all types of Lewis acid sites have adjacent OH groups. A good correlation between the number of Lewis acid sites and the intensity of perturbed OH groups is observed. Rehydration of the pyridine-adsorbed 450 °C pre-dehydrated alumina leads to removal of the pyridine adsorbed on the strong and medium strong Lewis acid sites and recovery of the surface of the original alumina. Water cannot replace the pyridine adsorbed on the weak Lewis acid sites.

A joint experimental–theoretical study on trimethylphosphine adsorption on the Lewis acidic sites present in TS-1 zeolite

31P MAS NMR desorption experiments and density functional calculations were combined to study the interactions between TS-1 zeolite and probe molecules. Two types of Lewis acidic sites at −34.2 and −32.0 ppm in the 31P MAS NMR spectra were identified, which correspond to the Ti(OSiO 3) 4 and (OSiO 3) 3Ti(OH) species with the Ti atoms at the T12 sites. Trimethylphosphine (TMP) was chemisorbed on the Lewis acidic sites with the adsorption energies calculated to be −6.2 and −19.6 kJ mol −1, respectively. On all the other T sites, TMP was physisorbed with slightly positive adsorption energies, in the range 0.82–5.92 kJ mol −1. The two types of Lewis acidic sites are responsible for the catalytic reaction of TMP oxidation, with their catalytic activities explained by structural and Mulliken charge analyses. It was also found that further hydrolysis of Si–O–Ti links to form (SiO) 2Ti(OH) 2 or (SiO)Ti(OH) 3 species were not observed in TS-1 zeolite.

Determination of the catalytically active oxidation Lewis acid sites in Sn-beta zeolites, and their optimisation by the combination of theoretical and experimental studies

Two types of Lewis acid sites are shown to exist in Sn-beta zeolite: partially hydrolysed framework tin centers ( Si O ) 3Sn OH (sites A) and fully framework coordinated tin atoms Sn( Si O ) 4 (sites B). ONIOM and DFT calculations for adsorbed acetonitrile, which is a probe molecule for Lewis acid sites, show that acetonitrile coordinates more strongly to partially hydrolysed framework Sn (sites A) in Sn-beta zeolite than to nonhydrolysed framework Sn (sites B). The corresponding IR bands associated with the ν(C N) stretching vibration were also calculated. Adsorption–desorption experiments of deuterated acetonitrile on Sn-beta give two IR bands at 2316 and 2308 cm −1 with IR shifts of 43 and 51 cm −1 with respect to gas-phase acetonitrile. These values are in excellent agreement with theoretical calculations, making it possible then to associate the Lewis sites related to 2316 and 2308 cm −1 with partially hydrolysed and nonhydrolysed framework Sn sites, respectively. Catalytic results confirm that partially hydrolysed Sn (sites A) is much more active than fully framework-integrated Sn (sites B) for Baeyer–Villiger oxidation of cyclic ketones. Finally, the ratio between these two types of sites could be influenced by postsynthesis treatments.

Improved Description of the Surface Acidity of η-Alumina

The surface acidity of an activated eta-alumina catalyst has been investigated by examining the interaction of pyridine with the catalyst by a combination of gravimetric and volumetric adsorption isotherms, infrared spectroscopy (diffuse reflectance and transmission), inelastic neutron scattering spectroscopy, temperature-programmed desorption spectroscopy, and gravimetric desorption experiments. From previous work, this surface was considered to contain three types of Lewis acid sites of increasing acidity: weak, medium, and strong. However, this multitechnique approach reveals the presence of an additional type of Lewis acid site. Although the traditional pyridine ring modes about 1580 cm(-1) are consistent with previous studies, temperature-programmed infrared spectroscopy of the surface hydroxyl groups and mass-selective temperature-programmed desorption experiments establish that the medium-strength Lewis acid category can be subdivided into two components. In this way, the surface structure of the activated catalyst is redefined as comprising (i) weak, (ii) medium-weak, (iii) medium-strong and (iv) strong Lewis acid sites. The (O-H) stretching mode of surface hydroxyl groups provides information on the local structure of the distinct sites, and schematic descriptions for these sites are proposed.

Mesoporous titania: effect of thermal treatment on the texture and acidic properties

Mesoporous titania materials have been synthesized by using urea as low cost template via sol–gel reactions of titaniumisopropoxide, followed by removing the urea by extraction with water. They were characterized by FT-IR, XRD, DTA-TGA and BET. The S BET of the solids increases with the increase of the urea content and decreases with the increase of the calcination temperature. The XRD patterns of the samples exhibit only the characteristic peaks of the anatase phase. The crystallinity increased when the temperature went up. The acidic characteristics of the samples were evaluated by TPD and DRIFTS of adsorbed pyridine. The total amount of pyridine adsorbed decreases in parallel with the increase of the calcination temperature. The samples displayed two types of Lewis acid sites, with which pyridine interacts via coordination bonding.

Acid–base properties of supported gallium oxide catalysts

The acid–base properties of supported gallium oxide catalysts have been investigated by microcalorimetry and FTIR spectroscopy of pyridine (acidity) and CO 2 (basicity) adsorption. Pyridine adsorption microcalorimetry has shown that loading supports such as γ-Al 2O 3, TiO 2, and ZrO 2 with gallium oxide in a surface concentration close to the theoretical monolayer, resulted in a decrease of the surface acidity of the catalysts compared to that of the supports, while, in the case of SiO 2, new Lewis acid sites were created. The number of acid sites for the supported and bulk gallium oxide catalysts was in the following order: Ga 2O 3/SiO 2<Ga 2O 3/ZrO 2<Ga 2O 3/TiO 2≈Ga 2O 3<Ga 2O 3/Al 2O 3. This order is related to the degree of dispersion of gallium oxide on the surface of each support and to the interaction between guest and host oxides. Two types of Lewis acid sites were identified by pyridine adsorption, corresponding to the support and the supported gallium oxide, respectively. The catalysts displayed very weak basicity, except for the samples supported on zirconia and alumina.

An FTIR Study of the Surface Acidity of USY Zeolites: Comparison of CO, CD3CN, and C5H5N Probe Molecules

A comparison of three basic probe molecules of varying strength, namely pyridine, acetonitrile, and carbon monoxide, was made, and their suitability toward the characterization of surface acidity in zeolitic materials was assessed. Two test samples were employed: both ultrastable Y (USY) zeolites but with differing silica:alumina ratios and degree of extraframework material. Carbon monoxide proved to be the most selective probe, not only being able to differentiate Bronsted (BAS) from Lewis acid sites (LAS), but BAS with varying acid strength as well. However, its weak basicity did not allow interaction with OH groups situated within the sodalite cages, this only being achieved with the two more basic probes. OH groups interacting with extraframework material also failed to form H-bonds with CO. Both pyridine and acetonitrile interacted with all OH groups, and pyridine was protonated by the most acidic bridged species. Although LAS could be told apart from BAS, differentiation of different types of LAS a...

Structure and Lewis acid properties of gallia–alumina catalysts

The surface structure of Ga 2O 3–Al 2O 3 catalysts was investigated by IR and ESR spectroscopy. As a result of modification by gallia, alumina hydroxyl cover changes significantly. The new band 3680–3688 cm −1 ascribed to bridging structure GaOHAl has been observed. By means of IR spectroscopy, adsorbed CO and ESR with anthraquinone as a probe molecule, two types of Lewis acid sites and their concentration were defined. These sites are coordinately unsaturated aluminium cation Al cus 3+ and coordinately unsaturated gallium cation Ga cus 3+. In the case of Ga 2O 3–Al 2O 3 catalysts with a gallia content of 1–10 mol%, surface Lewis acid properties are in accordance with the assumption of solid solution formation. Only under significantly non-equilibrium conditions (large excess of dopant — 20 mol% Ga 2O 3–Al 2O 3, relatively low temperature of calcination −720 K, short time of interaction — 4 h) it becomes possible to stabilize highly dispersed state of gallium oxide, for which the characteristic feature is high concentration and local regulation of Ga cus 3+ on the surface. It was shown that on the alumina surface there are favourable (more than in the case of pure gallia) conditions for the Ga cus 3+ formation.

FTIR study of the interaction of some branched aliphatic molecules with the external and internal sites of H-ZSM5 zeolite

The interaction of the branched molecules pivalonitrile (2,2-dimethylpropionitrile, PN), methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA), and 2,2-dimethylbutane (DMB), as well as of methylcyclohexane, benzonitrile and benzene over different H-ZSM5 zeolite samples was investigated by FTIR spectroscopy. PN does not enter the cavities of MFI structures at room temperature and 0–25 Torr pressure, at least when Al content is quite high. Also, TBA and MTBE do not enter the cavities under these conditions although isobutene produced by their decomposition, easily enters and polymerizes in the internal acidic sites. Conversely, DMB slowly enters the MFI zeolite cavities. On the other hand, the co-presence of benzene allows PN to enter the cavities and to interact with the internal sites. This shows that the access of single molecules can be influenced by other molecules in a mixture. The experiments also confirmed that two kinds of terminal silanol groups, indistinguishable from the point of view of the OH stretching band but differing in their Bronsted acidity, are located at the external surface of the ZSM5 zeolite crystals. Additionally, it was established that the external ZSM5 zeolite surface, even for low Al content samples, contains two types of Lewis acid sites. Conversely, no evidence was found for the presence of bridging Si–(OH)–Al sites at the external surface of the ZSM5 zeolite samples. This further corroborates the proposal that the bridging “ zeolitic” Bronsted acid sites are actually formed as a consequence of the porous structure of the zeolites and do not simply arise from the substitution of Al for silicon in a silica framework.

Role of mordenite acid properties in silver cluster stabilization

Abstract In the present work, the role of SiO 2 /Al 2 O 3 molar ratio in silver cluster stabilization inside mordenite pores was studied. It was found that SiO 2 /Al 2 O 3 molar ratio regulates silver cluster stabilization by means of change of mordenite acid properties. The high concentration and high strength of Bronsted acid sites favor the stabilization of silver in the form of clusters inside the pores. In contrast, low concentration and low strength of Bronsted acid sites and high concentration of Lewis sites inside the channels can hinder stabilization of silver clusters. In this case, large particles on the external surface of zeolite are preferably formed. It was revealed that silver cluster introduction in its turn influences acid properties of mordenites, changing the concentration and structure of acid sites. As a result of silver incorporation, four types of Lewis acid sites presented in H-mordenites disappear and a new type of Lewis acid site appears. For the majority of samples, it is accompanied by enhancement of concentration of Lewis sites so that all Al becomes ‘IR visible’.

Paramagnetic complexes of the probe molecules with electron accepting sites on the surface of M 2O 3–ZrO 2: (M=Al,Ga)

The electron accepting and Lewis acid properties of dehydroxylated surfaces of M 2O 3–ZrO 2 (M=Al, Ga) were investigated. The investigation was carried out by EPR with 2,2,6,6-tetramethylpiperidine- N-oxyl (TEMPO) and anthraquinone probe molecules. With the help of TEMPO two types of Lewis acid sites on the Al 2O 3–ZrO 2 (Ga 2O 3–ZrO 2) surface were defined: coordinatively unsaturated aluminium cation Al 3+ cus or gallium cation Ga 3+ cus and coordinatively unsaturated zirconium cation Zr 4+ cus. It was found out that electron accepting sites which are able to ionize anthraquinone are presented on the zirconia and Al 2O 3–ZrO 2 (Ga 2O 3–ZrO 2) surfaces as well. In the case of Al 2O 3–ZrO 2 obtained results can be explained with the presence of amorphous Al 2O 3 that covered microcrystals of metastable tetragonal zirconia. In the case of Ga 2O 3–ZrO 2 surface Lewis acid properties are in accordance with the assumption of solid solution formation.

Probing the surface acidity of modified γ-alumina with1H and15N high-resolution solid state NMR spectroscopy

1H MAS NMR and15N NMR studies of adsorbed N2 and N2O molecules were used to characterize Bronsted and Lewis acidic sites of unmodified γ-Al2O3 and γ-Al2O3 modified with NaOH. Changes in the concentrations of surface hydroxyls with the increase in the number of more “basic” OH groups for NaOH/γ-Al2O3 have been found by1H MAS NMR experiments. Two different types of Lewis acidic sites in γ-Al2O3 have been revealed using15N NMR studies. The strongest sites are poisoned even at small NaOH concentrations (ca. 0.05 wt.%). Not only the number of electron-accepting sites but also their strength are supposed to decrease for modified γ-alumina.

Systematic Comparison of a Saponite Clay Pillared with Al and Zr Metal Oxides

Alumina and zirconia pillared Ballarat saponites have been prepared by cation exchange of the saponite in aluminum chlorhydrate and zirconyl chloride solutions, respectively. The nature of the pillaring species and the method of preparation have been varied in order to examine their effect on the amount of occluded oxide, as well as the thermal stability, surface area, and acidic properties of the resulting pillared clays. Higher reaction (cation exchange) temperature (80 °C) enhances the degree of polymerization, increases the mobility of the pillaring species, and results in greater incorporation between the sheets of the saponite. In general, it is observed that the alumina-pillared saponites have a higher thermal stability than their zirconia-pillared analogues and retain an ordered layered structure even after calcination at 700 °C. Infrared spectroscopy of adsorbed pyridine suggests the presence of Bronsted acid sites and two types of Lewis acid sites (of different strength) for the alumina-pillared materials. Only single types of Bronsted and Lewis acid sites, however, were detected for the zirconia-pillared saponites. The thermal desorption of cyclohexylamine and dehydration of pentan-1-ol were used as further probes to monitor the acidity of the different pillared saponites.

DRFT-IR Studies of the Surface of γ-Alumina

γ-Alumina surfaces dehydrated at 450 and 700 °C were studied by using the conventional pyridine adsorption-DRFTIR technique. Three types of Lewis acid sites are present on the surface, and in each case pyridine adsorbed on them is concomitantly interacting with their adjacent OH groups. Because only three types of Lewis acid sites are present, only three hydrogen bonding bands were observed upon pyridine adsorption. From correlation of OH bands with hydrogen bonding bands, it is found that weak Lewis acid sites have type II66 or II64 OH groups as their neighbors, medium strong Lewis acid sites have type III OH groups near by, and strong Lewis acid sites have type I6 OH groups next to them. Dehydration at 700 °C generates more types of OH groups that also interact with pyridine. The reactivity of OH groups is not determined by their acidity and space restrictions, but rather by their proximity to Lewis acid sites. We assigned the three types of Lewis acid sites to five-, four-, and three-coordinate Al3+ ions.

Synthesis and characterization of various MgO and related systems

Various magnesium oxide catalysts have been prepared by thermal treatment of two different precursors: Mg(OH)2 and Mg5(OH)2(CO3)4. An additional system based on MgO and doped with B2O3 was also prepared. The textural and acid–base properties of the catalysts were investigated. The synthesized solids were characterized from adsorption isotherms, X-ray diffraction (XRD), 1H MAS NMR, diffuse reflectance IR Fourier transform spectroscopy (DRIFT) and temperature-programmed desorption–mass spectrometry (TPD–MS) of adsorbed probe molecules (pyridine, 2,6-dimethylpyridine and carbon dioxide). The surface properties of the solids were found to be strongly influenced by the magnesium oxide preparation conditions (viz. the precursor and calcination method used). Use of Mg(OH)2 as the precursor and in vacua calcination provided highly basic solids (those with the highest proportions of strong basic sites). Various types of Lewis-acid sites were observed in the catalysts prepared in vacuo, probably as the result of the presence of Mg2+ cations of low coordination after the calcination.

Acidity of sulfated zirconia as studied by FTIR spectroscopy of adsorbed CO and NH3 as probe molecules

The acid properties of pure and sulfated zirconias were studied by FTIR spectroscopy of adsorbed CO and NH3 as probe molecules. Whereas pure monoclinic zirconia shows only Lewis acidity, sulfation of tetragonal and monoclinic zirconia creates new bridging OH groups. Two types of Bronsted-acidic centers and two types of Lewis-acidic centers with enhanced acid strength were identified. A communication between the different types of Lewis-acidic sites and the related adsorbed sulfate molecules could be shown. The coordination of basic molecules such as CO onto Lewis-acid sites induces a decrease of the intrinsic Bronsted-acidity of the bridging OH groups. These effects are discussed on the basis of a model of the acidic centers that was previously proposed.

31P and119Sn high resolution solid state CP/MAS NMR study of Al2O3-SnO2 systems

A series of Al2O3-SnO2 catalysts with the mole ratio of Al2O3 to SnO2 equal to 1:1, 1 ∶ 0.5, 1 ∶ 0.1, 1 ∶ 0.05 and 1 ∶ 0.01 were characterized by31P NMR of adsorbed trimethyl phosphine (TMP) and119Sn MAS NMR spectroscopy. It was found from31P NMR that no Bronsted acid sites exist in these samples. Pure SnO2 shows two different types of Lewis acid sites; in the mixed oxide samples a Lewis peak characteristic of pure Al2O3 is always seen, together with either one or two other Lewis peaks, depending on the Sn concentration.119Sn CP/MAS NMR spectra of the highest Sn-content sample show one narrow line at −603 ppm superimposed on a very broad line, indicating a strong interaction between Al and Sn oxides.